All the downvotes here kinda got me legit angry. Incurious fools and jokers.
It's not a complete answer, but it's partially because the development of Chrome and Firefox have always been highly competitive resulting in them both adopting rapid release cycles around the same time in the early 2010's.
I haven't read too much into the topic, but I wouldn't be surprised if this was as much a marketing decision as well as a developer one. Similar to how Microsoft didn't want to release an XBox 2 in competition with a PlayStation 3.
https://en.m.wikipedia.org/wiki/Firefox_version_history https://en.m.wikipedia.org/wiki/Google_Chrome#Development
These are just the Wikipedia links, but there is interesting discussion of development history to be had, here.
That's my disclaimer that my research on the topic was less than exhaustive when I posted it at midnight, ~~smartass~~cool guy. I then when on to offer a legitimate, if simple answer with sources that I linked. I see now the error of my ways in trying to provider a sincere answer to a question instead of posting the same tired dunk as everyone else.
I have learned the error of my ways and will carry this lesson with me into the future as we build this Lemmy community.